(1) Field of the Invention
The present invention relates to (a) a method of cutting a substrate on which semiconductor laser devices made of nitride semiconductors and the like are formed and a method of coating the end faces of the cut substrates, and (b) a semiconductor laser device manufacturing apparatus which is capable of executing these methods.
(2) Description of the Related Art
A Group III nitride semiconductor (represented as InGaAlN in general) is expected to be a material capable of realizing a blue violet semiconductor laser device which functions as a light source in a next-generation high-density optical disc. In the research and development stage, the performances of a blue violet laser device made of a GaN system nitride semiconductor has reached the level which satisfies almost all of the specifications of the next generation optical disc system such as a blue ray disc system. At present, the research and development has been carried out continuously and actively in order to realize a high output performance and improve the reliability.
In general, a very hard substrate such as a sapphire substrate and a SiC substrate is used for realizing a crystal growth of a nitride semiconductor. A method of forming a device structure by epitaxially growing a semiconductor layer on the substrate according to the MOCVD (Metal Organic Chemical Vapor Deposition) method is used. In these days, a GaN substrate can be obtained by dividing or removing a thick film crystal which has been grown on the host substrate from the host substrate according to the HVPE (Hydride Vapor Phase Epitaxy) method, and therefore a device structure has been formed on such a GaN substrate. However, in any case, it is very difficult to obtain a planarized cleavage planes which are necessary for forming the resonator mirror of a semiconductor laser device because the substrate is much harder than a conventional semiconductor substrate made of Si, GaAs and the like. Conventionally, line-shaped guides are formed on a substrate made of sapphire, SiC or the like using, for example, a diamond scriber, and the substrate is cleaved by pressing an edge jig on the line-shaped guides. In this case, it is difficult to obtain planarized cleavage planes with a high reproducibility. This increases the threshold current of the resulting semiconductor laser device, and causes the problem of deteriorating the process yield. Here, it is general that a dielectric multi-layer film mirror is coated on each cleavage plane for the purpose of reducing the threshold current by improving the reflectivity of the end face.
Example methods of forming cleavage planes and coating end faces of a conventional nitride semiconductor laser device will be described below (refer to page 122 of “Wakaru Handotai no Kiso to Ohyo (Basis and applications of semiconductor laser)”, written by Shoji Hirata, published by CQ Press).
FIG. 1 is a flow chart showing methods of forming cleavage planes and coating end faces of conventional nitride semiconductor laser devices.
First, in a so-called cleaving apparatus, straight scribe lines are formed on the back side of a sapphire substrate, the nitride semiconductor wafer is cut into bar-shaped wafers by pressing an edge jig on these scribe lines, and the new end faces of the wafers are exposed so as to form cleavage planes (Step S901). Here, on the sapphire substrate a GaN system semiconductor laser structure has been formed through an epitaxial growth according to, for example, the MOCVD method, and these scribe lines are formed on the back side, on which no epitaxial growth layer has been formed, of the sapphire substrate.
Second, the bar-shaped wafers are loaded on a jig intended for high-frequency (RF: Radio Wave) sputtering in ambient atmosphere (Step S902). Here, the bar-shaped wafers are placed so that so-called front end faces which are the light emitting planes of the semiconductor laser devices face upward.
Third, the bar-shaped wafers loaded on the jig are loaded on an RF sputtering apparatus, and a dielectric coating lamination film such as a SiO2/TiO2 multi-layer film is formed on one of the end faces, of each wafer, exposed by the cutting of the original wafer (Step S903). In this way, a coating film is deposited on the so-called front end faces which are the light emitting planes.
Forth, the bar-shaped wafers are extracted from the RF sputtering apparatus in ambient atmosphere, turned upside down and reloaded on the jig for RF sputtering so that the so-called rear end faces which are the planes, which do not emit light, of the semiconductor laser device face upward (Step S904).
Fifth, the bar-shaped wafers are loaded on the RF sputtering apparatus again, and a dielectric coating lamination film such as a SiO2/TiO2 multi-layer film is deposited on the rear end face of each wafer exposed by the cutting of the original wafer (Step S905). In this way, the coating film is deposited on the rear end face of each wafer. Here, the thickness of the dielectric film is determined so that the reflectivity of the front end face becomes approximately 10 percent and the reflectivity of the rear end face becomes approximately 90 percent.
Lastly, the bar-shaped wafers are extracted, and cut into chip-shaped wafers so as to form semiconductor laser devices (Step S906).